clarify manual description of array assignment. fixes JuliaLang#4727

doc/manual/arrays.rst

@@ -172,9 +172,8 @@ The result X generally has dimensions
 ``(i_1, i_2, ..., i_n)`` of X containing the value
 ``A[I_1[i_1], I_2[i_2], ..., I_n[i_n]]``. Trailing dimensions indexed with
 scalars are dropped. For example, the dimensions of ``A[I, 1]`` will be
-``(length(I),)``. The size of a dimension indexed by a boolean vector
-will be the number of true values in the vector (they behave as if they were
-transformed with ``find``).
+``(length(I),)``. Boolean vectors are first transformed with ``find``; the size of
+a dimension indexed by a boolean vector will be the number of true values in the vector.
 
 Indexing syntax is equivalent to a call to ``getindex``::
 
@@ -208,9 +207,13 @@ where each I\_k may be:
 3. An arbitrary integer vector, including the empty vector ``[]``
 4. A boolean vector
 
-The size of X should be ``(length(I_1), length(I_2), ..., length(I_n))``, and
-the value in location ``(i_1, i_2, ..., i_n)`` of A is overwritten with
-the value ``X[I_1[i_1], I_2[i_2], ..., I_n[i_n]]``.
+If ``X`` is an array, its size must be ``(length(I_1), length(I_2), ..., length(I_n))``,
+and the value in location ``i_1, i_2, ..., i_n`` of ``A`` is overwritten with
+the value ``X[I_1[i_1], I_2[i_2], ..., I_n[i_n]]``. If ``X`` is not an array, its
+value is written to all referenced locations of ``A``.
+
+A boolean vector used as an index behaves as in ``getindex`` (it is first transformed
+with ``find``).
 
 Index assignment syntax is equivalent to a call to ``setindex!``::